1. Field of the Invention
The present invention relates to video display. More particularly, the present invention relates to a method and apparatus for displaying video at an optimum frame rate.
2. Discussion of Related Art
A conventional video sequence includes a series of frames of still images. The frames appear as a continuous motion video to the human eye when displayed at a minimum rate of 24 frames/second. Conventional video display devices such as televisions and computer monitors display individual frames at a xe2x80x9crefresh ratexe2x80x9d. Typically the refresh rate is higher than 24 frames/second. For example, televisions display video at 30 frames/second.
Frames can be displayed in progressive mode, i.e., line by line, beginning with a top line and sequentially proceeding to the bottom line. A frame can be divided into its even and odd lines, the even and odd lines being commonly known as respective xe2x80x9cevenxe2x80x9d and xe2x80x9coddxe2x80x9d fields, and the even and odd fields displayed sequentially (so called interlace mode). The human eye does not perceive the alternating display of even and odd lines of a frame as long as at least 24 frames/second are displayed.
A conventional technique for encoding 24 Hz frames for display at 30 Hz interlaced is 3:2 pulldown (hereafter xe2x80x9c3:2 formatxe2x80x9d). In 3:2 format, four fields of a video are displayed sequentially in five fields.
FIG. 1 schematically depicts an example of display of Frames 1-3, in interlaced mode, as Fields 1-5 in accordance with the 3:2 format. For each of input Frames 1-3, field 1 represents a top field, i.e., a field that includes a top displayed line and is either an even or odd field and field 2 represents the complement of field 1. As shown in FIG. 1, field 1 of a first frame, Frame 1, is first displayed as Field 1, followed by field 2 of Frame 1 (Field 2). Then field 1 and field 2 of Frame 2 are displayed as respective Fields 3 and 4. Field 1 of Frame 2 is then repeated as Field 5 after the display of field 2 of Frame 2. Thus with 3:2 format, a first field of every other frame is repeated. Because of the repeat of the top field of every other frame, one drawback with 3:2 format is that every other frame is noticeably longer in duration (so called xe2x80x9cjutterxe2x80x9d).
A time stamp associated with each input frame indicates when the associated field and frame is to be displayed relative to a clock signal. Hereafter the term xe2x80x9csegmentxe2x80x9d is used to refer to a field or a frame. Displaying each segment at a time set by its time stamp ensures that displayed video speed is as intended. The time stamps are set with respect to a specified system clock frequency. In 3:2 format the time stamp of the first field of every other frame is skewed to reflect that the even field of every other frame is repeated.
Thus what is needed is a method and apparatus that eliminates jutter and displays video at a frame rate of optimum video appearance.
An embodiment of the present invention includes a frame rate converter that receives segments having an input frame rate and provides segments at a rate of N times the frame rate, where the segment is selected from a group consisting of a frame or field, the frame rate converter including: a storage device which stores the segments, where the segments includes a first, second, and third segments; and a display device coupled to receive the segments from the storage device and to provide the first, second, and third segments, where the display device provides the first segment and then provides the second segment following completion of providing the first segment where the second segment is previously available or otherwise again provides the first segment.
In an embodiment, the frame rate converter further includes a decoder device, where the third segment comprises a top first field derived from a frame encoded in 3:2 format and where the decoder device adjusts a time stamp of the third segment.
The present invention will be more fully understood in light of the following detailed description taken together with the accompanying drawings.